1. Field of the Invention
This invention relates to a semiconductor avalanche photodiode (referred to hereinafter as an APD). More particularly, it relates to an APD which achieves both low noise and high speed operation.
2. Description of the Related Art
Long-distance and high-capacity optical transmission using a wavelength of 10 to 1.7 .mu.m, where the optical transmission loss of a silica optical fiber becomes minimum, has been increasing in practical use. As an optical detector for this type of transmission, many types of APD devices have been developed. A typical one of these APD devices is disclosed in U.S. Pat. Application Ser. No. 037,000, filed Apr. 10, 1987 by the assignee of the subject application. In this prior application, as schematically illustrated in FIG. 1, the numeral 1 denotes an n.sup.+ -type GaSb substrate; the numeral 2 denotes an n-type GaSb light absorbing layer; the numeral 3 denotes an n-type Al.sub.x Ga.sub.1-x Sb avalanche multiplication layer, where x=0.065; the numeral 4A denotes an n-type Al.sub.x Ga.sub.1-x Sb window layer, where x=0.3, having a band gap energy larger than that of the avalanche multiplication layer 3; and the numeral 5 denotes a p.sup.+ -type region doped in the window layer 4A for forming a pn junction. Some thickness of the window layer 4A is left between the pn junction 5' and the avalanche multiplication layer 3, and a light to be detected is irradiated through the p.sup.30 -type region 5 into the light absorbing layer 2. The above-described material of the avalanche multiplication layer 3 generates a high ionization rate of positive holes to that of electrons by a resonant impact ionization phenomenon, resulting in the achievement of both low noise and high speed operation of the APD. The resonant impact ionization is described by o Hilderbrand et al. in IEEE Journal of Quantum Electronics, Vol. QE-17, No. 2, p. 284-288, February, 1981.
The problems with the above-described prior art APD are discussed below. In FIG. 2, the quantum efficiency characteristic which is an average number of electrons or positive holes photoelectrically emitted in a photoelectric material per incidental photons (i.e., a unit intensity of light) thereto versus the incidental wavelength, is presented. Incidental light having a 1.3 .mu.m wavelength produces both types of carriers, i.e., electrons and positive holes, in the avalanche multiplication layer 3 for x=0.065, as indicated by a notation "M", even though the light should pass therethrough without producing carriers therein. The most important aspect of employing resonant impact ionization to reduce the noise generation is obtained when only positive carriers are injected from the light absorbing layer 2 into the avalanche multiplication layer 3. Accordingly, the undesirable generation of both types of carriers (i.e., electrons and positive holes), in the multiplication layer 3 deteriorates the pure generation of the resonant impact ionization. As a result, the low noise characteristics of the APD are deteriorated. The same problem still exists for 1.55 .mu.m wavelength incident light as indicated by a notation "M'", though the amount of the produced carriers is much less.
Another prior art APD is disclosed in Japanese Unexamined Patent Publication Sho 60-105281 which is assigned to the assignee of the subject application and which is schematically illustrated in FIG. 3. In FIG. 3, the numeral 3-11 denotes a p.sup.+ -type GaSb substrate, and the numeral 3-12 denotes a p-type Ga.sub.0.935 Al.sub.0.065 Sb avalanche multiplication layer which generates a resonant impact ionization phenomenon. The numeral 3-13 denotes a p-type Ga.sub.0.9 Al.sub.0.1 Sb light absorbing layer, and the numeral 3-14 denotes a p-type Ga.sub.0.6 Al.sub.0.4 Sb window layer. The numeral 3-15 denotes an n.sup.+ -type region doped in the window layer 3-14 for forming a pn junction, and the numeral 3-18 denotes an anti-reflection layer. The numerals 3-19 and 3-20 denote electrodes for applying a bias voltage to the APD. The light to be detected is injected through the n.sup.+ region 3-15 into the light absorbing layer 3-13. Accordingly, the light does not have to pass through the avalanche multiplication layer 3-12. Therefore, the problem of the prior art APD of FIG. 1 is avoided. However, other problems are created therein. Because of the longer distance between the pn junction 3-16 and the avalanche multiplication layer 3-12, any fluctuations in the impurity density or the thickness, etc., of the layers therebetween in the fabrication process seriously affect the APD characteristics. Furthermore, currently available diffusion source materials, such as sulfur or selenium, for forming the n region 3-15, are difficult to work with to achieve an adequate diffusion depth, resulting in difficulties in designing and fabricating the n.sup.30 region 3-15.
A third prior art APD is schematically illustrated in FIG. 4, where the same or like numerals denote the same or corresponding portions of the APD of FIG. 1. A substrate I, and a light absorbing layer 2 are the same as those of FIG. 1. An avalanche multiplication layer 3' is formed of the same material in the same way as the FIG. 1 APD. However, the layer 3' is formed in a mesa shape. A peripheral portion of the mesa is filled with the same material as that of the window layer 4A of FIG. 1, so as to form a guard ring 4B. Into the surfaces of the mesa-shaped layer 3' and an adjacent part of the guard ring 4B, zinc is diffused approximately 1 .mu.m deep to form a p.sup.30 -type region 5. The p.sup.30 -type region 5 forms a pn junction 5' with the avalanche multiplication layer 3' as well as the guard ring 4B. The pn junction also has a peripheral portion 5". A light to be detected is injected through the p.sup.30 -type region 5 into the light absorbing layer 2. The disadvantage of the prior art APD of FIG. 4 is the same as that of the prior art APD of FIG. 1.